Glass powders, methods for producing glass powders and devices fabricated from same

ABSTRACT

Glass powders and methods for producing glass powders. The powders preferably have a small particle size, narrow size distribution and a spherical morphology. The method includes forming the particles by a spray pyrolysis technique. The method also includes making a glass layer on a substrate. The invention also includes novel devices and products formed from the glass powders.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/032,298 filed Dec. 21, 2001, which is adivisional application of U.S. patent application Ser. No. 09/141,394filed Aug. 27, 1998, now U.S. Pat. No. 6,360,562, which is acontinuation-in-part of U.S. patent application Ser. No. 09/030,057filed Feb. 24, 1998, now U.S. Pat. No. 6,338,809, and acontinuation-in-part of U.S. patent application Ser. No. 09/028,628filed Feb. 24,1 998, now U.S. Pat. No. 6,602,439, and acontinuation-in-part of U.S. patent application Ser. No. 09/028,029filed Feb. 24, 1 998, now abandoned. Each of the foregoing referencedpatent applications is incorporated by reference herein as if set forthbelow in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to glass powders having well controlledchemical and mechanical properties as well as methods for producingglass powders, and intermediate products and devices incorporating theglass powders. The glass powders are preferably produced by spraypyrolysis of glass precursors.

2. Description of Related Art

Many product applications require glass powders that have one or more ofthe following properties: spherical morphology; high purity; smallaverage size; narrow size distribution; controlled chemistry; and littleor no agglomeration. Examples of glass powder applications requiringsuch characteristics include, but are not limited to, thick-film pastesused for fabricating electronic devices. Thick-film pastes are mixturesof fine powders in an organic vehicle, wherein the organic vehicle isremoved after application of the paste to a substrate.

In particular, many product applications require glass powders having asmall average size, such as from about 0.1 μm to about 10 μm, and aspherical morphology. It can also be advantageous if the powder consistsof glass particles having a narrow size distribution without anysubstantial agglomeration of the particles. Most glass powders areproduced by forming a melt of the desired glass composition, quenchingthe molten glass and milling the resulting glass to reduce the particlesize of the glass. See, for example, U.S. Pat. No. 4,820,661 by Nairwhich discloses an aluminoborosilicate glass useful in thick-filmcompositions for crossover dielectrics. Such methods result in glasspowders having a jagged and irregular morphology, wide spread ofparticle size and other characteristics which are undesirable inprecision applications.

There have been attempts in the art to produce glass particles havingimproved chemical and physical characteristics. U.S. Pat. No. 4,775,520by Unger et al. discloses a process for forming monodispersed silica(SiO₂) particles having an average size of from about 0.05 to about 10μm. The particles, which are useful as sorption materials inchromatography, are formed by a two-step process including hydrolyticpolycondensation and the addition of a silane to control the reaction.

U.S. Pat. No. 5,173,457 by Shorthouse discloses a borosilicate glassuseful for thick-film applications having a size of less than 5 μm and aspherical morphology. The glass is formed by a sol-gel process.

U.S. Pat. No. 5,589,150 by Kano et al. discloses a silica gel formed bymilling a hydrogel slurry and spray drying the slurry to form gelparticles having an average size of 30 to 100 μm and a sphericalmorphology. The gel particles are useful as polymer catalyst carriers.

Typically, sol-gel and related precipitation routes for forming glassesare limited in their usefulness. The precursors, such as alkoxides, areprohibitively expensive and the process cannot easily be converted to acontinuous production method. It is also difficult to produce complexglass compositions with good homogeneity of the different glasscomponents. Further, it can be difficult to separate the glass particlesfrom the liquid in which they are produced.

The continued miniaturization and increased complexity of electroniccomponents has created a need for materials, including glass particles,with well-controlled physical and chemical characteristics. For example,thick-film paste technology must continue to meet the demands ofdecreased line width and decreased pitch, i.e., decreased distancebetween traces. As a result, pastes have been developed that have aphoto-imaging capability to enable the formation of traces having adecreased width and pitch. In this process, a photoactive thick-filmpaste is applied to a substrate and the paste is then dried and exposedto ultraviolet light through a photomask and the exposed portions of thepaste are developed to remove unwanted portions of the paste.

Examples of such photoactive pastes are disclosed in: U.S. Pat. No.3,958,996 by Inskip; U.S. Pat. No. 4,119,480 by Nishi et al.; U.S. Pat.No. 4,598,037 by Felten; U.S. Pat. No. 4,613,560 by Dueber et al.; andU.S. Pat. Nos. 5,032,478 and 5,032,490 both by Nebe et al. Each of theforegoing U.S. Patents are incorporated herein by reference in theirentirety.

Glass compositions for thick-film pastes are also useful for formingdielectric layers in electronic circuits. For example, U.S. Pat. No.4,820,661 by Nair discloses an aluminoborosilcate glass useful forcrossover dielectrics. U.S. Pat. No. 4,959,330 by Donohue et al.discloses a crystallizable glass including ZnO and BaO that is useful asa dielectric layer. U.S. Pat. No. 5,210,057 by Haun et al. discloses analkaline earth zinc silicate glass that is partially crystallizable andis useful for forming dielectric layers. Each of the foregoing U.S.Patents are incorporated herein by reference in their entirety.

To meet the demands of these and similar applications, glass powders,particularly complex glass powders, having well-controlled physical andchemical characteristics are required. To date, such glass powders havenot been provided.

It would be particularly advantageous to provide a flexible productionmethod capable of producing glass powders which would enable controlover the powder characteristics as well as the versatility toaccommodate complex glass compositions which are either difficult orimpossible to produce using existing production methods. For example, itwould be advantageous to provide control over the particle size,particle size distribution, morphology, homogeneity, and porosity of theglass powder. It would be particularly advantageous if such glasspowders could be produced in large quantities on a substantiallycontinuous basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 3 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 4 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 5 is a side view of the transducer mounting plate shown in FIG. 4.

FIG. 6 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 4.

FIG. 7 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 8 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 9 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 10 is a side view of the liquid feed box shown in FIG. 9.

FIG. 11 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 12 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 13 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 14 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 15 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 16 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 15.

FIG. 17 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 18 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 19 is a side view of the gas manifold shown in FIG. 18.

FIG. 20 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 21 is a side view of the generator lid shown in FIG. 20.

FIG. 22 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 23 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 24 is a front view of a flow control plate of the impactor shown inFIG. 23.

FIG. 25 is a front view of a mounting plate of the impactor shown inFIG. 23.

FIG. 26 is a front view of an impactor plate assembly of the impactorshown in FIG. 23.

FIG. 27 is a side view of the impactor plate assembly shown in FIG. 26.

FIG. 28 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 29 is a top view of a gas quench cooler of the present invention.

FIG. 30 is an end view of the gas quench cooler shown in FIG. 29.

FIG. 31 is a side view of a perforated conduit of the quench coolershown in FIG. 29.

FIG. 32 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 33 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 34 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 35 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 36 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIG. 37 illustrates an SEM photomicrograph of a glass powder producedaccording to an embodiment of the present invention.

FIG. 38 illustrates a particle size distribution for a glass powderbatch according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to glass powders and methodsfor producing glass powders. The invention is also directed to novelintermediate products and devices fabricated using the glass powders. Asused herein, glass powders or glass particles are inorganic materialsthat are predominately amorphous, as determined, for example, by x-raydiffraction analysis of the powder. Glasses are characterized by arandom structure with no long-range (crystalline) order. Finely powderedglass powders are sometimes referred to as glass frits or fillers.

The present invention is particularly applicable to complex glasses. Asused herein, complex glasses are those that include at least onestructural forming oxide (e.g. SiO₂) and at least one additional oxide(e.g. B₂O₃, PbO). Complex glasses include binary, ternary or quaternaryglasses, as well as glasses including more than four components.Examples of particularly preferred complex glasses are disclosed indetail hereinbelow.

In one aspect, the present invention provides a method for preparing aparticulate product including a glass. A feed of liquid-containing,flowable medium, including at least one precursor for the desired glassmaterial, is converted to aerosol form, with droplets of the mediumbeing dispersed in and suspended by a carrier gas. Liquid from thedroplets in the aerosol is then removed to permit formation in adispersed state of the desired particles. Typically, the feed precursoris pyrolyzed in a furnace to make the particles. In one embodiment, theparticles are subjected, while still in a dispersed state, tocompositional or structural modification, if desired. Compositionalmodification may include, for example, coating the particles. Structuralmodification may include, for example, annealing the particles. The termpowder is often used herein to refer to the particulate product of thepresent invention. The use of the term powder does not indicate,however, that the particulate product must be dry or in any particularenvironment. Although the particulate product is typically manufacturedin a dry state, the particulate product may, after manufacture, beplaced in a wet environment, such as in a paste or slurry.

The process of the present invention is particularly well suited for theproduction of finely divided glass particles having a small weightaverage size. In addition to making particles within a small weightaverage particle size, the particles may be produced with a narrow sizedistribution, thereby providing size uniformity that is desired for manyapplications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying chemical composition wherein theparticles are high purity with good chemical homogeneity. Complexglasses, such as binary, ternary or quaternary glasses, can be formed bythe method. The ability to tightly control the chemical composition ofthe glass particles advantageously permits control over the propertiesof the glass such as glass transition temperature (T_(g)), dielectricconstant, density, and the like.

The glass particles may also include a second-phase that is not a glass.For example, one phase (e.g. a crystalline oxide) may be uniformlydispersed throughout a matrix of another phase (e.g. a glass).Alternatively, one phase may form an interior core while another phaseforms a coating that surrounds the core. Other morphologies are alsopossible, as is discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least oneprecursor for the desired particles, and a carrier gas 104 are fed to anaerosol generator 106 where an aerosol 108 is produced. The aerosol 108is then fed to a furnace 110 where liquid in the aerosol 108 is removedto produce glass particles 112 that are dispersed in and suspended bygas exiting the furnace 110. The glass particles 112 are then collectedin a particle collector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, such as colloidal silica particles, thoseparticles should be relatively small in relation to the size of dropletsin the aerosol 108. Such suspended particles should typically be smallerthan about 1 μm in size, preferably smaller than about 0.5 μm in size,and more preferably smaller than about 0.3 μm in size and mostpreferably smaller than about 0.1 μm in size. Most preferably, thesuspended particles should be able to form a colloid. The suspendedparticles could be finely divided particles, or could be agglomeratemasses comprised of agglomerated smaller primary particles. For example,0.5 μm particles could be agglomerates of nanometer-sized primaryparticles. When the liquid feed 102 includes suspended particles, theparticles preferably comprise not greater than about 15 weight percentof the liquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Preferably, theprecursor will be a material dissolved in a liquid solvent of the liquidfeed 102, such as a salt, e.g., a nitrate salt. The precursor can alsobe an acid, such as boric acid (H₃BO₃), a precursor to B₂O₃. Theprecursor may undergo one or more chemical reactions in the furnace 110to assist in production of the particles 112. Alternatively, theprecursor material may contribute to formation of the particles 112without undergoing chemical reaction. This could be the case, forexample, when the liquid feed 102 includes, as a precursor material,suspended particles that are not chemically modified in the furnace 110.In any event, the particles 112 comprise at least one componentoriginally contributed by the precursor.

For the production of complex glass powders, the liquid feed 102 willtypically include multiple precursor materials, which may be presenttogether in a single phase or separately in multiple phases. Forexample, the liquid feed 102 may include multiple precursors in solutionin a single liquid vehicle. Alternatively, one precursor material couldbe in a solid particulate phase (e.g. colloidal silica) and a secondprecursor material could be in a liquid phase (e.g. a metal salt). Also,one precursor material could be in one liquid phase and a secondprecursor material could be in a second liquid phase, such as could bethe case when the liquid feed 102 comprises an emulsion. One of theadvantages of the present invention is that high quality glass powderscan be produced from reasonably inexpensive precursor materials.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form. Thecarrier gas 104 may be inert, in that the carrier gas 104 does notparticipate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the glass particles 112. For example, oxygencan be a critical reactive component to the formation of the glassparticles.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may also include nonliquid material, such as one ormore small particles held in the droplet by the liquid. For example,when producing glass composite particles, one phase of the particles maybe provided in the liquid feed 102 in the form of suspended precursorparticles and a second phase of the particles may be produced in thefurnace 110 from one or more precursors in the liquid phase of theliquid feed 102. Furthermore the precursor particles could be includedin the liquid feed 102, and therefore also in droplets of the aerosol108, for the purpose only of dispersing the particles for subsequentcompositional or structural modification during or after processing inthe furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size and narrow sizedistribution. In this manner, the glass particles 112 may be produced ata desired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is preferably capable of producing the aerosol108 such that it includes droplets having a weight average size in arange having a lower limit of about 1 μm and preferably about 2 μm; andan upper limit of about 20 μm; preferably about 10 μm, more preferablyabout 7 μm and most preferably about 5 μm. A weight average droplet sizein a range of from about 2 μm to about 4 μm is preferred for manyapplications. The aerosol generator is also preferably capable ofproducing the aerosol 108 such that it includes droplets in a narrowsize distribution. Preferably, the droplets in the aerosol are such thatat least about 70 weight percent (more preferably at least about 80weight percent and most preferably at least about 85 weight percent) ofthe droplets are smaller than about 10 μm and more preferably at leastabout 70 weight percent (more preferably at least about 80 weightpercent and most preferably at least about 85 weight percent) aresmaller than about 5 μm. Furthermore, preferably no greater than about30 weight percent, more preferably no greater than about 25 weightpercent and most preferably no greater than about 20 weight percent, ofthe droplets in the aerosol 108 are larger than about twice the weightaverage droplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.25 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the glass particles 112. The maximum average streamtemperature, or reaction temperature, refers to the maximum averagetemperature that an aerosol stream attains while flowing through thefurnace. This is typically determined by a temperature probe insertedinto the furnace.

For the production of glass particles, residence time in the heatingzone of the furnace 110 will depend on the composition of the glassparticles. In one embodiment, the residence time in the heating zone isat least about 4 seconds, with shorter than about 15 seconds beingpreferred. The residence time will depend on the reaction temperature aswell as the geometric size of the reactor and the carrier gas flow rate.The residence time should be long enough, however, to assure that theparticles 112 attain the desired maximum stream temperature for a givenheat transfer rate such that substantially all of the precursors arefully reacted. In that regard, with extremely short residence times,higher furnace temperatures could be used to increase the rate of heattransfer so long as the particles 112 attain a maximum temperaturewithin the desired stream temperature range. However, the temperatureshould not be so high that volatile species (e.g. PbO) are lost. Thus,that mode of operation is not typically preferred. Also, it is preferredthat, in most cases, the maximum stream temperature not be attained inthe furnace 110 until substantially at the end of the heating zone inthe furnace 110. For example, the heating zone will often include aplurality of heating sections that are each independently controllable.The maximum stream temperature should typically not be attained untilthe final heating section, and more preferably until substantially atthe end of the last heating section. This is important to reduce thepotential for thermophoretic losses of material. Also, it is noted thatas used herein, residence time refers to the actual time for a materialto pass through the relevant process equipment. In the case of thefurnace, this includes the effect of increasing velocity with gasexpansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Further, theaccumulation of liquid at sharp edges can result in re-release ofundesirably large droplets back into the aerosol 108, which can causecontamination of the particulate product 116 with undesirably largeparticles. Also, over time, such liquid collection at sharp surfaces cancause fouling of process equipment, impairing process performance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, fused silica or alumina. Alternatively, the tube may bemetallic. Advantages of using a metallic tube are low cost, ability towithstand steep temperature gradients and large thermal shocks,machinability and weldability, and ease of providing a seal between thetube and other process equipment. Disadvantages of using a metallic tubeinclude limited operating temperature and increased reactivity in somereaction systems. For example, some metal tubes can out-gas chromium atincreased temperatures and very small amounts of chromium (e.g. aslittle as 150 ppm) can discolor the glass particles. Given theforegoing, the proper tube can be selected for a particular glasscomposition and reactor temperature. For making high purity glassparticles, fused silica (quartz) tubes are often preferred.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture. When higher temperatures are required,ceramic tubes are typically used. One major problem with ceramic tubes,however, is that the tubes can be difficult to seal with other processequipment, especially when the ends of the tubes are maintained atrelatively high temperatures.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingglass particles 112 to produce the particulate product 116. Oneembodiment of the particle collector 114 uses one or more filters toseparate the glass particles 112 from the gas. Such a filter may be ofany type, including a bag filter. Another preferred embodiment of theparticle collector uses one or more cyclones to separate the particles112. A cyclone is preferred according to one embodiment of the presentinvention due to the ability of a cyclone to separate the glass powderbased upon particle size. Thus, the collected particles canadvantageously have an even narrower particle size distribution. Otherapparatus that may be used in the particle collector 114 include anelectrostatic precipitator. Collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the glass particles 112 are suspended. Also, collection shouldnormally be at a temperature that is low enough to prevent significantagglomeration of the glass particles 112, that is, the temperatureshould be below the softening point of the glass.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 2, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 2, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 3, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 2, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 2, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferablywithin 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 2 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.2, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extends the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such a coating can significantly extend the transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer discs 120 are preferably also at an elevatedtemperature in the ranges discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 4-21 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 4 and5, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 2.

As shown in FIGS. 4 and 5, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 6,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 2.

A preferred transducer mounting configuration, however, is shown in FIG.7 for another configuration for the transducer mounting plate 124. Asillustrated in FIG. 7, an ultrasonic transducer disc 120 is mounted tothe transducer mounting plate 124 by use of a compression screw 177threaded into a threaded receptacle 179. The compression screw 177 bearsagainst the ultrasonic transducer disc 120, causing an o-ring 181,situated in an o-ring seat 182 on the transducer mounting plate, to becompressed to form a seal between the transducer mounting plate 124 andthe ultrasonic transducer disc 120. This type of transducer mounting isparticularly preferred when the ultrasonic transducer disc 120 includesa protective surface coating, as discussed previously, because the sealof the o-ring to the ultrasonic transducer disc 120 will be inside ofthe outer edge of the protective seal, thereby preventing liquid frompenetrating under the protective surface coating from the edges of theultrasonic transducer disc 120.

Referring now to FIG. 8, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 4-5). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 4-5) when the bottomretaining plate 128 is mated with the transducer mounting plate 124 tocreate a volume for a water bath between the transducer mounting plate124 and the bottom retaining plate 128. The openings 184, therefore,provide a pathway for ultrasonic signals generated by ultrasonictransducers to be transmitted through the bottom retaining plate.

Referring now to FIGS. 9 and 10, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 8), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 9 and 10 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 8). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 9-11, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 1l. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 12, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 12are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 12,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 12, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 2, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 12 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 13, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 11. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.14. As shown in FIG. 14, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 15 and 16. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 16,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 15 and 16 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 17, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 18 and 19, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 11). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 18 and 19, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 20 and 21, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 9 and 10). The generator lid140, as shown in FIGS. 20 and 21, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

It is important that the aerosol stream that is fed to the furnace 110have a high droplet flow rate and high droplet loading as would berequired for most industrial applications. With the present invention,the aerosol stream fed to the furnace preferably includes a droplet flowof greater than about 0.5 liters per hour, more preferably greater thanabout 2 liters per hour, still more preferably greater than about 5liters per hour, even more preferably greater than about 10 liters perhour, particularly greater than about 50 liters per hour and mostpreferably greater than about 100 liters per hour; and with the dropletloading being typically greater than about 0.04 milliliters of dropletsper liter of carrier gas, preferably greater than about 0.083milliliters of droplets per liter of carrier gas 104, more preferablygreater than about 0.167 milliliters of droplets per liter of carriergas 104, still more preferably greater than about 0.25 milliliters ofdroplets per liter of carrier gas 104, particularly greater than about0.33 milliliters of droplets per liter of carrier gas 104 and mostpreferably greater than about 0.83 milliliters of droplets per liter ofcarrier gas 104.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. However, theprocess of the present invention can be enhanced by further classifyingby size the droplets in the aerosol 108 prior to introduction of thedroplets into the furnace 110. In this manner, the size and sizedistribution of particles in the particulate product 116 are furthercontrolled.

Referring now to FIG. 22, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 22, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 23-27.

As seen in FIG. 23, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 24. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 25. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 24).

Referring now to FIGS. 26 and 27, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 23).

The configuration of the impactor plate 302 shown in FIG. 22 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm, more preferably to remove dropletslarger than about 10 μm, even more preferably to remove droplets of asize larger than about 8 μm and most preferably to remove dropletslarger than about 5 μm. The droplet classification size in the dropletclassifier is preferably smaller than about 15 μm, more preferablysmaller than about 10 μm, even more preferably smaller than about 8 μmand most preferably smaller than about 5 μm. The classification size,also called the classification cut point, is that size at which half ofthe droplets of that size are removed and half of the droplets of thatsize are retained. Depending upon the specific application, however, thedroplet classification size may be varied, such as by changing thespacing between the impactor plate 302 and the flow control plate 290 orincreasing or decreasing aerosol velocity through the jets in the flowcontrol plate 290. Because the aerosol generator 106 of the presentinvention initially produces a high quality aerosol 108, having arelatively narrow size distribution of droplets, typically less thanabout 30 weight percent of liquid feed 102 in the aerosol 108 is removedas the drain liquid 284 in the droplet classifier 288, with preferablyless than about 25 weight percent being removed, even more preferablyless than about 20 weight percent being removed and most preferably lessthan about 15 weight percent being removed. Minimizing the removal ofliquid feed 102 from the aerosol 108 is particularly important forcommercial applications to increase the yield of high qualityparticulate product 116. It should be noted, however, that because ofthe superior performance of the aerosol generator 106, it is typicallynot required to use an impactor or other droplet classifier to obtain asuitable aerosol. This is a major advantage, because the addedcomplexity and liquid losses accompanying use of an impactor may oftenbe avoided with the process of the present invention.

With some applications of the process of the present invention, it maybe possible to collect the glass particles 112 directly from the outputof the furnace 110. More often, however, it will be desirable to coolthe glass particles 112 exiting the furnace 110 prior to collection ofthe particles 112 in the particle collector 114. Referring now to FIG.28, one embodiment of the process of the present invention is shown inwhich the particles 112 exiting the furnace 110 are sent to a particlecooler 320 to produce a cooled particle stream 322, which is then fed tothe particle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 29-31, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 31, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 29-31, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 29, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 29-31, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 32, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for glass powder batches having aweight average size of larger than about 1 μm, although a series ofcyclones may be needed to obtain the desired degree of separation.Cyclone separation is particularly preferred for powders having a weightaverage size of larger than about 1.5 μm.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the glass particles 112exiting the furnace. Most commonly, the compositional modification willinvolve forming on the glass particles 112 a material phase that isdifferent than that of the particles 112, such as by coating the glassparticles 112 with a coating material. One embodiment of the process ofthe present invention incorporating particle coating is shown in FIG.33. As shown in FIG. 33, the glass particles 112 exiting from thefurnace 110 go to a particle coater 350 where a coating is placed overthe outer surface of the glass particles 112 to form coated particles352, which are then sent to the particle collector 114 for preparationof the particulate product 116. Coating methodologies employed in theparticle coater 350 are discussed in more detail below.

With continued reference primarily to FIG. 33, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process. Whencoating particles that have been premanufactured by a different route,such as by liquid precipitation, it is preferred that the particlesremain in a dispersed state from the time of manufacture to the timethat the particles are introduced in slurry form into the aerosolgenerator 106 for preparation of the aerosol 108 to form the dryparticles 112 in the furnace 110, which particles 112 can then be coatedin the particle coater 350. Maintaining particles in a dispersed statefrom manufacture through coating avoids problems associated withagglomeration and redispersion of particles if particles must beredispersed in the liquid feed 102 for feed to the aerosol generator106. For example, for particles originally precipitated from a liquidmedium, the liquid medium containing the suspended precipitated glassparticles could be used to form the liquid feed 102 to the aerosolgenerator 106. It should be noted that the particle coater 350 could bean integral extension of the furnace 110 or could be a separate piece ofequipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 34, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theglass particles 112. The particle modifier 360, therefore, typicallyprovides a temperature controlled environment and necessary residencetime to effect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the structure or morphology of the particles 112.For example, the particles 112 may be annealed in the particle modifier360 to densify the glass particles 112 or to recrystallize the glassparticles 112 into a polycrystalline form.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 35. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 35.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generation for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 μm, a spray nozzle atomizer may be preferred. For smaller-particleapplications, however, and particularly for those applications toproduce particles smaller than about 3 μm, as is generally desired withthe particles of the present invention, an ultrasonic generator, asdescribed herein, is particularly preferred. In that regard, theultrasonic generator of the present invention is particularly preferredfor when making particles with a weight average size of from about 0.1μm to about 3 μm.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 2-21,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 μm toabout 10 μm, preferably in a size range of from about 1 μm to about 5 μmand more preferably from about 2 μm to about 4 μm. Also, the ultrasonicgenerator of the present invention is capable of delivering high outputrates of liquid feed in the aerosol. The rate of liquid feed, at thehigh liquid loadings previously described, is preferably greater thanabout 25 milliliters per hour per transducer, more preferably greaterthan about 37.5 milliliters per hour per transducer, even morepreferably greater than about 50 milliliters per hour per transducer andmost preferably greater than about 100 millimeters per hour pertransducer. This high level of performance is desirable for commercialoperations and is accomplished with the present invention with arelatively simple design including a single precursor bath over an arrayof ultrasonic transducers. The ultrasonic generator is made for highaerosol production rates at a high droplet loading, and with a narrowsize distribution of droplets. The generator preferably produces anaerosol at a rate of greater than about 0.5 liter per hour of droplets,more preferably greater than about 2 liters per hour of droplets, stillmore preferably greater than about 5 liters per hour of droplets, evenmore preferably greater than about 10 liters per hour of droplets andmost preferably greater than about 40 liters per hour of droplets. Forexample, when the aerosol generator has a 400 transducer design, asdescribed with reference to FIGS. 4-21, the aerosol generator is capableof producing a high quality aerosol having high droplet loading aspreviously described, at a total production rate of preferably greaterthan about 10 liters per hour of liquid feed, more preferably greaterthan about 15 liters per hour of liquid feed, even more preferablygreater than about 20 liters per hour of liquid feed and most preferablygreater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as: $\begin{matrix}{{Re} = {({pvd})/\mu}} \\{{{{where}:\quad\rho} = {{fluid}\quad{density}}};} \\{{v = {{fluid}\quad{mean}\quad{velocity}}};} \\{{d = {{conduit}\quad{inside}\quad{diameter}}};{and}} \\{{\mu = {{fluid}\quad{{viscosity}.}}}\quad}\end{matrix}$It should be noted that the values for density, velocity and viscositywill vary along the length of the furnace 110. The maximum Reynoldsnumber in the furnace 110 is typically attained when the average streamtemperature is at a maximum, because the gas velocity is at a very highvalue due to gas expansion when heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second, and mostpreferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum stream temperature in the furnace and a point atwhich the aerosol has been cooled to an average stream temperature belowabout 200° C. is shorter than about 2 seconds, more preferably shorterthan about 1 second, and even more preferably shorter than about 0.5second and most preferably shorter than about 0.1 second. In mostinstances, the maximum average stream temperature attained in thefurnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains. In that regard, it is preferred that a drain beplaced as close as possible to the furnace inlet to prevent liquidaccumulations from reaching the furnace. The drain should be placed,however, far enough in advance of the furnace inlet such that the streamtemperature is lower than about 80° C. at the drain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet to be of a substantially constant cross-sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross-sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 36, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. Ifsufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 29-31, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 μm, this vertical flow should,preferably, be vertically upward. For larger-size particles, such asthose larger than about 1.5 μm, the vertical flow is preferablyvertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is preferably shorter than about 15 seconds, more preferablyshorter than about 10 seconds, even more preferably shorter than about 7seconds and most preferably shorter than about 5 seconds.

For the production of glass particles according to the presentinvention, the liquid feed 102 includes at least one metal oxideprecursor for preparation of the glass particles 112. The metal oxideprecursor may be a substance in either a liquid or solid phase of theliquid feed 102. Typically, the metal oxide precursor will be ametal-containing compound, such as a metal salt, dissolved in a liquidsolvent of the liquid feed 102. The metal oxide precursor may undergoone or more chemical reactions in the furnace 110 to assist inproduction of the glass particles 112. Alternatively, the metal oxideprecursor may contribute to formation of the glass particles 112 withoutundergoing chemical reaction. This could be the case, for example, whenthe liquid feed 102 includes suspended oxide particles as a precursormaterial, such as particulate silica.

The liquid feed 102 thus includes the chemical components that will formthe glass particles 112. For example, the liquid feed 102 can comprise asolution containing nitrates, acetates, chlorides, sulfates, hydroxides,or oxalates of a metal. Particularly preferred precursor salts includemetal nitrates and metal acetates. These salts are typically highlysoluble in water and the solutions maintain a low viscosity. Metalnitrates are even more preferred since they do not contain any carbonthat can potentially contaminate the end-product. It may be desirable toacidify the solution to increase the solubility, such as by addinghydrochloric acid.

The precursor solution can also include solid particulates, for example,the precursor solution can include particulate silica as a precursor fora silicate glass.

The solution preferably has a precursor concentration that isunsaturated to avoid the possibility of undesirable precipitateformation. The solution preferably includes a soluble precursor to yielda concentration of from about 1 to about 50 weight percent of the glasscomposition, more preferably from about 1 to 20 weight percent of theglass composition and even more preferably from about 3 to about 15weight percent of the glass composition, such as about 5 to 7.5 weightpercent of the glass composition. The final particle size of the glassparticles 112 is also influenced by the precursor concentration.Generally, lower precursor concentrations will yield glass particleshaving a smaller average particle size.

Preferably, the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable. As is disclosedabove, the pH of the aqueous-based solutions can be adjusted to alterthe solubility characteristics of the precursor in the solution or thestability of, for example, colloid particles in the precursor solution.In addition to the foregoing, the liquid feed 102 may also include otheradditives that contribute to the formation of the particles.

Thus, the liquid feed 102 may include multiple precursor materials,which may be present together in a single phase or separately inmultiple phases. For example, the liquid feed 102 may include multipleprecursors in solution in a single liquid vehicle. Alternatively, oneprecursor material could be in a solid particulate phase and a secondprecursor material could be in a liquid phase. Also, one precursormaterial could be in one liquid phase and a second precursor materialcould be in a second liquid phase, such as could be the case for whenthe liquid feed 102 comprises an emulsion.

A carrier gas 104 under controlled pressure is introduced to the aerosolgenerator to move the droplets away from the generator. The carrier gas104 may comprise any gaseous medium in which droplets produced from theliquid feed 102 may be dispersed in aerosol form. Also, the carrier gas104 may be inert, in that the carrier gas 104 does not participate information of the particles 112. Alternatively, the carrier gas 104 mayhave one or more active components that contribute to formation of theparticles 112. For the production of glass particles 112, the preferredcarrier gas includes air since it is a low cost gas that can supplysufficient oxygen to form the glass particles.

The carrier gas 104 carries the aerosol through a heated reaction zone,as is discussed above. According to the present invention, the reactiontemperature in the heating zone is preferably near the softening pointof the glass composition to produce a dense material. To produce porousand/or hollow materials, the temperature is preferably below the glasstransition temperature of the glass composition. Although the exacttemperature can vary for different glass compositions, it is generallypreferred that the reaction temperature is from about 300° C. to about1500° C., and more preferably from about 500° C. to about 800° C. Inmost instances, it is preferred that the temperature be at least about600° C. to ensure complete reaction of the particles.

Depending on the reaction temperature, the residence time in the heatingzone can vary. It is preferred however that the residence time be atleast about 2 seconds and typically no more than about 15 seconds. It isoften preferred to adjust the process parameters to accommodate longerresidence times at lower temperatures to ensure that volatile componentssuch as PbO do not volatilize from the glass composition.

To form substantially uniform coatings on the surface of the glassparticles, if desired, a reactive gas composition can be contacted withthe glass particles at an elevated temperature after the particles havebeen formed. For example, the reactive gas can be introduced into theheated reaction zone at the distal end so that the desired compound, forexample a metal, deposits on the surface of the particles.

More specifically, the droplets can enter the heated reaction zone at afirst end such that the droplets move through the heating zone and formthe glass particles. At the opposite end of the heating zone, a reactivegas composition can be introduced such that the reactive gas compositioncontacts the glass particles at an elevated temperature. Alternatively,the reactive gas composition can be contacted with the heated particlesin a separate heating zone located downstream from the heated reactionzone.

Coatings can be generated on the particle surface by a number ofdifferent mechanisms. One or more precursors can vaporize and fuse tothe hot particle surface and thermally react resulting in the formationof a thin-film coating by chemical vapor deposition (CVD). Preferredcoatings deposited by CVD include elemental metals. Further, the coatingcan be formed by physical vapor deposition (PVD) wherein a coatingmaterial physically deposits on the surface of the particles. Preferredcoatings deposited by PVD include organic materials and elementalmetals. Alternatively, the gaseous precursor can react in the gas phaseforming small particles, for example less than about 5 nanometers insize, which then diffuse to the larger particle surface and sinter ontothe surface, thus forming a coating. This method is referred to asgas-to-particle conversion (GPC). Whether such coating reactions occurby CVD, PVD or GPC is dependent on the reactor conditions, such astemperature, precursor partial pressure, water partial pressure and theconcentration of particles in the gas stream. Another possible surfacecoating method is surface conversion of the surface of the particles byreaction with a vapor phase reactant to convert the surface of the glassparticles to a different material than that originally contained in theparticles.

The coatings are preferably as thin as possible while maintainingconformity about particle such that the glass surface is notsubstantially exposed. For example, coatings can have an averagethickness of not greater than about 200 nanometers, preferably notgreater than about 100 nanometers, and more preferably not greater thanabout 50 nanometers. For most applications, the coating should have anaverage thickness of at least about 5 nanometers.

The structural modification that can occur in the particle modifier 360may be any modification to the structure or morphology of the particles112. For example, the particles 112 may be annealed in the particlemodifier 360 to densify the particles 112 or to crystallize the glassparticles 112 into a polycrystalline form. Also, the glass particles maybe annealed for a sufficient time to redistribute different materialphases within the particles 112 or to alter the thermal properties ofthe glass.

The present invention is directed to glass powder batches wherein theparticles constituting the powder batch preferably have a sphericalmorphology. Advantageously, the powders can also have a small averageparticle size and a narrow particle size distribution. It is preferredthat the powders are also substantially unagglomerated and have a highpurity. The powders according to the present invention are useful for anumber of applications including use in thick film pastes formicroelectronic applications.

The glass powder batches according to the present invention include acommercially useful quantity of glass particles. The glass particlespreferably include at least a first glass phase. The glass phase caninclude any glass composition and the particularly preferred glasscomposition will depend upon the application of the powder.

According to one embodiment, the glass particles preferably include atleast about 80 weight percent glass, and depending upon the application,preferably include at least about 90 weight percent glass and even morepreferably at least about 95 weight percent glass. In one preferredembodiment, the particles include at least about 99 weight percentglass, that is, not greater than about 1 weight percent of a crystallinephase.

Glass compositions can vary and include many components. The followingdescription of preferred glasses is by way of example, and is not meantto limit the present invention to specific glasses. The most commontypes of glasses are oxide glasses, which can generally be categorizedas: silicates, based on SiO₂, and including sub-groups such asaluminosilicates; borates, based on B₂O₃; phosphates, based on P₂O₅; andgermanates, based on GeO₂. The foregoing oxides are commonly referred toas the glass-formers. The structure of the glass can be modified throughthe addition of intermediate oxides, such as Al₂O₃, Bi₂O₃ and PbO. Athigh concentrations, these intermediate oxides can also be consideredglass-formers. Glass compositions can also be modified by the additionof one or more alkali (e.g., Li, Na, K, Rb, Cs) oxides and alkalineearth (e.g., Mg, Ca, Sr, Ba) oxides. Non-oxide glass compositions, suchas halide glasses and chalcogenide glasses, are used for specificapplications.

The present invention is particularly applicable to complex glasscompositions, which are those glass compositions that include at leasttwo components in non-trivial amounts, for example ternary andquaternary glass compositions.

The silicate-based glasses are the most common and are preferredaccording to the present invention. A particularly preferred glass forsome microelectronic applications are the dielectric borosilicateglasses, comprising at least SiO₂ and B₂O₃, such as the leadborosilicate glasses that also include PbO. An example of such a complexdielectric glass is given in Table I. TABLE I Typical Dielectric GlassComposition Component Range (wt. %) PbO 50-74 B₂O₃ 10-25 SiO₂  8-26Al₂O₃ 0-5 CaO 0-6 MgO 0-4 Na₂O 0-5

The weight percent of the individual components of the glass detailed inTable I can be selected to alter the properties of the glass, such asthe dielectric constant, thermal expansion coefficient, glass transitiontemperature and the like. Specific examples of borosilicate glasseswhich are useful for electronic applications include those disclosed inU.S. Pat. No. 4,613,560 by Dueber et al.; U.S. Pat. No. 5,032,478 byNebe et al.; U.S. Pat. No. 5,032,490 by Nebe et al.; and U.S. Pat. No.5,173,457 by Shorthouse. Each of the foregoing U.S. Patents disclosingborosilicate glass compositions is incorporated herein by reference intheir entirety.

Also preferred according to an embodiment of the present invention arethe aluminosilicate glasses which include at least SiO₂ and Al₂O₃. Atypical composition for an aluminosilicate glass is listed in Table II.TABLE II Typical Aluminosilicate Glass Composition Component Range (wt.%) SiO₂ 54-55 CaO 13-15 BaO 3-4 B₂O₃ 6-8 Al₂O₃ 20-22

A specific example of an aluminosilicate glass is disclosed in U.S. Pat.No. 4,598,037 by Felten. It will be appreciated by those skilled in theart that combinations of the foregoing glasses also occur in the art.For example, aluminoborosilicate glasses are known, as is disclosed inU.S. Pat. No. 4,820,661 by Nair and U.S. Pat. No. 5,173,457 byShorthouse. Each of the foregoing U.S. Patents disclosingaluminosilicate and alumino borosilicate glasses are incorporated hereinby reference in their entirety.

It is an advantage of the present invention that the glass compositionwithin the particles is homogeneous and well mixed on the atomic leveland has substantially no phase segregation of the different phases inthe particle. Such a high degree of homogeneity in complex glasses isoften not obtainable by traditional forming methods, such as sol-gel orliquid precipitation. However, it may be desirable for some applicationsthat the particles consist of two or more distinct phases, and such acomposition can also be formed according to the present invention.

Typically, the complex glass composition will be formed from a liquidsolution which includes both a glass-former precursor (e.g. SiO₂) and aprecursor for the intermediate oxides and/or glass modifiers. The weightpercentage of the different components can be adjusted by changing therelative ratios of precursors in the liquid precursor solution.

The glass powders according to one embodiment of the present inventioninclude glass particles having a small average particle size. Althoughthe preferred average size of the particles will vary according to theparticular application of the powder, the weight average particle sizeof the particles is at least about 0.05 μm, preferably is at least about0.1 μm and more preferably is at least about 0.3 μm. Further, accordingto this embodiment, the average particle size is preferably not greaterthan about 10 μm. More preferably the weight average particle size isnot greater than about 5 μm, particularly not greater than about 3 μm.

Although such small average particle sizes are preferred for someapplications, the present invention is also applicable to glass powdershaving a larger average particle size, such as up to about 20 μm. Suchglass powders can advantageously be produced according to the presentinvention using, for example, a nozzle-type atomizer to produce anaerosol stream with increased aerosol droplet size.

According to a preferred embodiment of the present invention, the powderbatch of glass particles has a narrow particle size distribution, suchthat the majority of glass particles are about the same size.Preferably, at least about 80 weight percent and more preferably atleast about 90 weight percent of the particles are not larger than twicethe weight average particle size. Thus, when the average particle sizeis about 2 μm, it is preferred that at least about 80 weight percent ofthe particles are not larger than 4 μm. Further, it is preferred that atleast about 80 weight percent of the particles are not larger than about1.5 times the weight average particle size. In a more preferredembodiment, at least about 90 weight percent of the particles are notlarger than 1.5 times the average particle size. Thus, when the averageparticle size is about 2 μm, it is preferred that at least about 80weight percent of the particles are not larger than 3 μm.

It is also possible according to the present invention to provide aglass powder batch having a bimodal particle size distribution. That is,the powder batch can include particles having two distinct and differentaverage particle sizes. A bimodal particle size distribution can enhancethe packing efficiency of the powder.

The glass powders produced by the processes described herein, namelyspray pyrolysis, can form soft agglomerates as a result of theirrelatively high surface energy (compared to larger particles). It isalso known to those skilled in the art that soft agglomerates may bedispersed easily by treatments such as exposure to ultrasound in aliquid medium or sieving. The particle size distributions describedherein are measured by mixing samples of the powders in a medium such aswater with a surfactant and a short exposure to ultrasound througheither an ultrasonic bath or horn. The ultrasonic treatment suppliessufficient energy to disperse the soft agglomerates into primaryspherical particles. The primary particle size distribution is thenmeasured by light scattering in a Microtrac instrument. This provides agood measure of the useful dispersion characteristics of the powderbecause this simulates the dispersion of the particles in a liquidmedium such as a paste or slurry that is used to deposit the particlesin a device. Thus, the references to particle size herein refer to theprimary particle size, such as after lightly dispersing the softagglomerates of the powder.

The glass particles produced according to the present invention alsohave a high degree of purity and it is preferred that the particlesinclude not greater than about 0.1 atomic percent impurities and morepreferably not greater than about 0.01 atomic percent impurities. Sinceno milling of the particles is required to achieve small averageparticle sizes, there are substantially no undesired impurities such asalumina, zirconia or high carbon steel in the powder batch. According toone preferred embodiment, the glass particles include less than about100 ppm, more preferably less than 50 ppm, of metallic impurities thatcan discolor the glass, such as chromium.

The formation of hollow particles is common in spray pyrolysis. In thepresent invention, it has been found that the formation of hollowparticles can be controlled through the selection of precursors,precursor concentration, pyrolysis temperature and residence time.According to one embodiment of the present invention, the glassparticles are dense (e.g. not hollow or porous), as measured by heliumpycnometry. According to this embodiment, the glass particles have aparticle density of at least about 80% of the theoretical value, morepreferably at least about 90% of the theoretical value and even morepreferably at least about 95% of the theoretical value. In oneembodiment, the particle density is at least about 99% of thetheoretical value. The theoretical density can be easily calculated forglasses based on the relative percentages of each component. Highdensity particles provide many advantages over porous particles,including reduced shrinkage during sintering and improved flowproperties.

According to another embodiment, however, the glass particles are hollowspheres having a reduced density. As is discussed above, such hollowparticles can be produced, for example, by reducing the reactiontemperature during manufacture to below the glass transition temperature(Tg) of the glass. Hollow particles can also be produced by carefulselection of the precursors. Such hollow particles are useful inelectronic applications requiring a low dielectric constant, such as adielectric constant of less than about 2.

The glass particles according to a preferred embodiment of the presentinvention are also substantially spherical in shape. That is, theparticles are not jagged or irregular in shape. Spherical particles areparticularly advantageous because they are able to disperse more readilyin a paste or other liquid medium and impart advantageous flowcharacteristics to compositions containing the particles.

In addition, the glass powder according to the present invention has alow surface area. The particles are substantially spherical, whichreduces the total surface area for a given mass of powder. Further, theelimination of larger particles from the powder batch eliminates theporosity that is typically associated with open pores on the surface ofsuch larger particles. Due to the elimination of the larger particles,the powder advantageously has a lower surface area. Surface area istypically measured using the BET nitrogen adsorption method which isindicative of the surface area of the powder, including the surface areaof accessible pores on the surface of the particles. For a givenparticle size distribution, a lower value of surface area per unit massof powder generally indicates solid or non-porous particles. Thereactivity of powders having a low surface area is reduced. Thischaracteristic can advantageously extend the shelf life of such powders.Preferably, the glass powders have a surface area that is close, such aswithin about 5 percent, of the calculated geometric surface area whichis calculated for monodispersed spheres having the same average particlesize as the glass powder.

In addition, the powder batches of glass particles according to thepresent invention are substantially unagglomerated, that is, theyinclude substantially no hard agglomerates of the glass particles. Hardagglomerates are physically coalesced lumps of two or more particlesthat behave as one large particle. Hard agglomerates are disadvantageousin most applications, particularly when the glass powder is applied to asubstrate in a liquid vehicle, such as a thick film paste. It ispreferred that no more than about 1.0 weight percent of the glassparticles in the powder batch of the present invention are in the formof hard agglomerates. More preferably, no more than about 0.5 weightpercent of the particles are in the form of hard agglomerates. In theevent that hard agglomerates do form, they can optionally be broken up,such as by jet-milling the powder.

According to one embodiment of the present invention, the glassparticles are composite glass particles, wherein the individualparticles include at least a first glass phase and at least a secondphase associated with the glass phase. The second phase can be, forexample, a metal. Preferred metals are the noble metals such as gold orsilver. Such composites can be produced by adding a salt of the metal tothe precursor solution, such as silver nitrate.

According to another embodiment of the present invention, the glassparticles are coated particles that include a particulate coating ornon-particulate (film) coating that substantially encapsulates the outersurface of the particles. Preferably, the coating is very thin and hasan average thickness of not greater than about 200 nanometers, morepreferably not greater than about 100 nanometers, and even morepreferably not greater than about 50 nanometers. While the coating isthin, the coating should substantially encapsulate the entire particlesuch that substantially no glass surface is exposed. Accordingly, thecoating preferably has an average thickness of at least about 5nanometers.

The coating can be a metal or other inorganic compound, or can be anorganic compound. For example, the particles can be coated with a metalto utilize the surface properties of the metal coating. The particlescan include more than one coating, if multiple coatings are desirable.

Further, a dielectric coating, either organic or inorganic, can be usedto achieve the appropriate surface charge characteristics to carry outdeposition processes such as electrostatic deposition, discussedhereinbelow.

The glass particles of the present invention can advantageously becoated with an organic compound, for example a surfactant to provideimproved dispersion which will result in smoother prints having lowerlump counts when applied as a paste. The organic compound for coatingthe particles can be selected from organic compounds such as PMMA(polymethylmethacrylate), polystyrene or the like. The organic coatingpreferably has an average thickness of not greater than about 100nanometers, and more preferably not greater than about 50 nanometers.The organic coating is substantially dense and continuous about theparticle.

The coating can also be comprised of one or more monolayer coatings,such as from about 1 to 3 monolayer coatings. A monolayer coating isformed by the reaction of an organic or an inorganic molecule with thesurface of the particles to form a coating layer that is essentially onemolecular layer thick. In particular, the formation of a monolayercoating by reaction of the surface of the particle with a functionalizedorgano silane such as halo- or amino-silanes, for examplehexamethyidisilazane or trimethylsilylchloride, can be used to modifythe hydrophobicity and hydrophilicity of the powders. Such coatingsallow for greater control over the dispersion characteristics of thepowder in a wide variety of paste compositions.

The monolayer coatings can also be applied to glass powders that havealready been coated with an organic or inorganic coating thus providingbetter control over the corrosion characteristics (through the use ofthe thicker coating) as well as dispersibility (through the monolayercoating) of the particles.

The glass powder batches according to the present invention are usefulin a number of applications and can be used to fabricate a number ofnovel devices and intermediate products. Such devices and intermediateproducts are included within the scope of the present invention.

The glass powders according to the present invention are particularlyuseful in microelectronic applications, including data processingapplications and advanced display applications. Complex glasses,particularly borosilicate glasses, are commonly used as dielectricmaterials in microelectronic circuits. For such applications, the glassshould have a low dielectric constant, good thermal expansion match tothe substrate, a well controlled glass transition temperature (T_(g))and low dielectric loss.

Glass powders can be deposited onto device surfaces or substrates by anumber of different deposition methods which involve the directdeposition of the dry powder such as dusting, electrophotographic orelectrostatic precipitation, while other deposition methods involveliquid vehicles such as ink jet printing, liquid delivery from asyringe, micro-pens, toner, slurry deposition, paste-based methods andelectrophoresis. In all these deposition methods, the powders describedin the present invention show a number of distinct advantages overpowders produced by other methods. For example, small, spherical, narrowsize distribution glass particles are more easily dispersed in liquidvehicles, they remain dispersed for a longer period and allow printingof smoother and finer features compared to powder made by alternativemethods.

Some glasses are also used in metal thick-film paste compositions tocontrol the shrinkage of the paste during sintering and facilitate thebonding of the paste to the substrate. Generally, the glass should havegood electrical resistivity, thermal shock resistance, good mechanicalstrength, good dielectric strength and low dielectric loss.

In the thick-film paste process, a viscous paste that includes afunctional particulate phase (e.g. a metal powder and/or a dielectricglass) is screen printed onto a substrate. More particularly, a porousscreen fabricated from stainless steel, polyester, nylon or similarinert material is stretched and attached to a rigid frame. Apredetermined pattern is formed on the screen corresponding to thepattern to be printed. For example, a UV sensitive emulsion can beapplied to the screen and exposed through a positive or negative imageof the design pattern. The screen is then developed to remove portionsof the emulsion in the pattern regions.

The screen is then affixed to a screen printing device and the thickfilm paste is deposited on top of the screen. The substrate to beprinted is then positioned beneath the screen and the paste is forcedthrough the screen and onto the substrate by a squeegee that traversesthe screen. Thus, a pattern of traces and/or pads of the paste materialis transferred to the substrate. The substrate with the paste applied ina predetermined pattern is then subjected to a drying and firingtreatment to solidify and adhere the paste to the substrate.

Thick film pastes have a complex chemistry and generally include afunctional phase, a binder phase and an organic vehicle phase. Thefunctional phase include metal powders which provide conductivity. Thebinder phase can be, for example, a mixture of metal oxide or glass fritpowders such as those according to the present invention. PbO basedglasses are commonly used as binders. The function of the binder phaseis to control the sintering of the film and assist the adhesion of thefunctional phase to the substrate and/or assist in the sintering of thefunctional phase. Reactive compounds can also be included in the pasteto promote adherence of the functional phase to the substrate.

Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins and other organics whose mainfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetates, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters.

The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste.Typically, the thick film paste will include from about 5 to about 95weight percent such as from about 60 to 85 weight percent, of thefunctional phase.

Examples of thick film pastes are disclosed in U.S. Pat. Nos. 4,172,733;3,803,708; 4,140,817; and 3,816,097 all of which are incorporated hereinby reference in their entirety.

Some applications of thick film pastes require higher tolerances thancan be achieved using standard thick-film technology, as is describedabove. As a result, some thick film pastes have photo-imaging capabilityto enable the formation of lines and traces with decreased width andpitch. In this type of process, a photoactive thick film paste isapplied to a substrate substantially as is described above. The pastecan include, for example, a liquid vehicle such as polyvinyl alcohol,that is not cross-linked. The paste is then dried and exposed toultraviolet light through a photomask to polymerize the exposed portionsof paste and the paste is developed to remove unwanted portions of thepaste. This technology permits higher density lines and other featuresto be formed. The combination of the foregoing technology with the glasspowders of the present invention permits the fabrication of devices withresolution and tolerances as compared to conventional technologies usingconventional glass powders.

In addition, a laser can be used instead of ultraviolet light through amask. The laser can be scanned over the surface in a pattern therebyreplacing the need for a mask. The laser light is of sufficiently lowintensity that it does not heating the glass or polymer above itssoftening point. The unirradiated regions of the paste can be removedleaving a pattern.

Likewise, conventional paste technology utilizes heating of a substrateto remove the vehicle from a paste and to fuse particles together ormodify them in some other way. A laser can be used to locally heat thepaste layer and scanned over the paste layer thereby forming a pattern.The laser heating is confined to the paste layer and drives out thepaste vehicle and heats the powder in the paste without appreciablyheating the substrate. This allows heating of particles, delivered usingpastes, without damaging a glass or even polymeric substrate.

Powders for use in thick-film pastes should have good dispersibility andflow properties. As is discussed above, the glass powders according tothe present invention are substantially spherical in shape and aresubstantially unagglomerated. Due to this unique combination ofproperties, the powders disperse and flow in a thick-film paste betterthan conventional powders which are not spherical and containagglomerates.

Other deposition methods for the powders can also be used. For example,a slurry method can be used to deposit the powder. The powder istypically dispersed in an aqueous slurry including reagents such aspotassium silicate and polyvinyl alcohol, which aids in the adhesion ofthe powder to the surface. For example, the slurry can be poured ontothe substrate and left to settle to the surface. After the powder hassedimented onto the substrate the supernatant liquid is decanted off andthe powder layer is left to dry.

Glass particles can also be deposited electrophoretically orelectrostatically. The particles are charged and are brought intocontact with the substrate surface having localized portions of oppositecharge. The layer is typically lacquered to adhere the particles to thesubstrate. Shadow masks can be used to produce the desired pattern onthe substrate surface.

Ink-jet printing is another method for depositing the glass powders in apredetermined pattern. The powder is dispersed in a liquid medium anddispensed onto a substrate using an inkjet printing head that iscomputer controlled to produce a pattern. The glass powders of thepresent invention having a small size, narrow size distribution andspherical morphology can be printed into a pattern having a high densityand high resolution. Other deposition methods utilizing a glass powderdispersed in a liquid medium include micro-pen or syringe deposition,wherein the powders are dispersed and applied to a substrate using a penor syringe and are then allowed to dry.

Patterns can also be formed by using an ink jet or micropen (smallsyringe) to dispense sticky material onto a surface in a pattern. Powderis then transferred to the sticky regions. This transfer can be done isseveral ways. A sheet covered with powder can be applied to the surfacewith the sticky pattern. The powder sticks to the sticky pattern anddoes not stick to the rest of the surface. A nozzle can be used totransfer powder directly to the sticky regions.

Many methods for directly depositing materials onto surfaces requireheating of the particles once deposited to sinter them together anddensify the layer. The densification can be assisted by including amolecular precursor to a material in the liquid containing theparticles. The particle/molecular precursor mixture can be directlywritten onto the surface using inkjet, micropen, and other liquiddispensing methods. This can be followed by heating in a furnace orheating using a localized energy source such as a laser. The heatingconverts the molecular precursor into the functional material containedin the particles thereby filling in the space between the particles withfunctional material.

Thus, the glass powders produced according to the present inventionresult in smoother powder layers when deposited by such liquid or drypowder based deposition methods. Smoother powder layers are the resultof the smaller average particle size, spherical particle morphology andnarrower particle size distribution compared to powders produced byother methods.

The glass powders of the present invention can also be used in resistorand/or thermistor component applications. For these applications, theglass powder is mixed with a conductive powder (e.g., a metal) in aspecified ratio to control the resistance of the component. For theseapplications, the resistivity and temperature coefficient of resistance(TCR) for the glass must be well-controlled. A common problem intraditional pastes for these applications is segregation of the metaland glass powders due to the large difference in particle size betweenthe two powders. The glass powder typically has an average size muchgreater than the average size of the metal powder. The glass powders ofthe present invention can advantageously have a size that is tailored tobe similar to the size of the metal powder, e.g. less than about 5 μm,resulting in a more uniform paste and improved component properties.Glasses commonly used in thick-film paste applications includeborosilicate glasses containing different amounts of modifiers such asAl₂O₃, Bi₂O₃, PbO, CdO, ZnO, BaO and CaO.

Structural applications of the glass powders according to the presentinvention include use as spacers for glass face-plates in displayapplications. The high tolerance of this application demands glasspowders with well-controlled physical properties such as a smallparticle size and narrow size distribution.

One preferred application of the glass powders of the present inventionis for the barrier ribs in a flat panel display, such as a plasmadisplay panel. Such barrier ribs provide electrical insulation and musthave well-controlled dielectric properties. Further, the ribs are narrowand have tightly controlled spacing in the device, therefore the powdersmust have well-controlled physical properties such as a small size and anarrow size distribution. The particles should also have a high purityso that the ribs are substantially transparent and do not discolor theviewing screen.

Other uses of glass powders can include high temperature/high pressurelubrication, such as for metal stamping. Glass powders are also used assealants wherein the glass (referred to as a solder glass) is selectedto have a lower softening point than the substrate glass. For example, asolder glass can be used to seal two glass plates together inliquid-crystal displays (LCD's). Other uses of the glass powders includedental applications, such as for crown and filling material wherein theglass is admixed with a ceramic and/or a resin. Glass powders in theform of beads or hollow micro spheres can also be used to deliver drugsor radiation into the body.

EXAMPLES

A complex glass precursor solution was prepared including colloidalparticulate silica, boric acid, lead acetate, zinc acetate and aluminumnitrate. The solution was atomized using ultrasonic transducers at afrequency of about 1.6 MHZ to produce an aerosol of precursor droplets.Air was used as a carrier gas to move the aerosol through an elongatedtubular furnace such that the droplets/particles had a residence time ofabout 10-12 seconds in the furnace.

The reaction temperature was varied to determine the effect of thereaction temperature on the formation of the particles. Reactiontemperatures were 500° C., 600° C., 650° C. and 700° C. At 500° C. and600° C., it was observed that the powder had a slightly tan color,indicating the presence of unreacted carbon from the acetate precursor.At 650° C. and 700° C., the powders were white. In all cases, thepowders were spherical and unagglomerated.

In a further set of experiments, the same precursor solutions (colloidalsilica, boric acid, lead acetate, zinc acetate and aluminum nitrate) ata total precursor concentration of 7.5 weight percent based on the glasscomposition were formed into an aerosol using a 7×7 array of ultrasonictransducers at a frequency of about 1.6 MHZ. A tubular furnace (36″×5.5″diameter) was used to heat the aerosol to the reaction temperature. Thetotal residence time was about 2-3 seconds. Temperatures ranged from400° C. to 1000° C. However, a fully reacted white powder was notobtainable. The color of the powder ranged from tan at lowertemperatures to bright yellow at higher temperatures. It is believedthat the residence time of 2-3 seconds was too short for the reaction ofthe acetates and complete elimination of carbon.

As a result, a precursor solution of nitrate precursors in distilledwater was formed. The precursor solution included colloidal silica(Cabot HS-5, Cabot Corporation, Mass.), aluminum nitrate, boric acid,lead nitrate and zinc nitrate. The precursor solution was formed into anaerosol using ultrasonic transducers at a frequency of about 1.6 MHZ.Air was used as a carrier gas at a flow rates of about 4 CFM in a 72inch by 5.5 inch heated tube, to yield residence times of about 4.5seconds. The temperature of the furnace was experimentally varied attemperatures of 600° C., 650° C. and 700° C.

At the lower temperatures, below 600° C., the powder was non-sphericaland angular, indicating unreacted nitrates were present. This wasconfirmed by x-ray diffraction. However, at 700° C. and a flow rate of 4CFM, the particles were fully reacted and spherical. This complex glasspowder is illustrated in FIG. 37 and the particles are spherical andunagglomerated. The particle size distribution is illustrated in FIG. 38for particles which were collected in a cyclone, which further narrowedthe size distribution of the powder. The volume average particle size ofthe powder was about 1 μm and there were substantially no particlesgreater than 2 μm. 90 percent of the particles had a size of less than1.3 μm, and 90 percent of the particles were at least 0.74 μm.

Thus, it is believed that nitrate precursors are preferred over acetateprecursors. The residence time of the aerosol should be sufficient toenable complete reaction of the precursors at a given temperature. Sincesome glass components, such as PbO, are highly volatile at hightemperatures, it is preferred to heat the aerosol at low temperaturesfor longer residence times.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. An aerosol method for making glass particles, the method comprising:generating an aerosol stream, as generated the aerosol stream comprisingdroplets comprising a liquid and at least one precursor for glassmaterial; and in the aerosol stream, forming glass particles comprisingthe glass material, the glass particles having a weight average particlesize of not greater than 5 microns.
 2. The method of claim 1, whereinthe at least one precursor comprises an acid.
 3. The method of claim 2,wherein the acid is boric acid.
 4. The method of claim 1, wherein the atleast one precursor comprises a salt.
 5. The method of claim 1, whereinthe at least one precursor comprises a metal salt.
 6. The method ofclaim 1, wherein the at least one precursor comprises a metal oxide. 7.The method of claim 1, wherein the at least one precursor comprises ametal nitrate.
 8. The method of claim 1, wherein the at least oneprecursor comprises a metal acetate.
 9. The method of claim 1, whereinthe at least one precursor comprises at least one member selected fromthe group consisting of metal chlorides, metal sulfates, metalhydroxides and metal oxalates.
 10. The method of claim 1, wherein theforming comprises heating the aerosol stream in a thermal reactor. 11.The method of claim 10, wherein the thermal reactor is a tubular furnacereactor.
 12. The method of claim 10, wherein the thermal reactor is aflame reactor.
 13. The method of claim 10, wherein the thermal reactoris a plasma reactor.
 14. The method of claim 1, wherein during theheating, the aerosol stream reaches a maximum average stream temperatureof greater than 800° C.
 15. The method of claim 1, wherein the dropletshave a weight average size in a range of from 1 micron to 20 microns.16. The method of claim 1, wherein the droplets have a weight averagesize in a range of from 1 micron to 10 microns.
 17. The method of claim1, wherein the droplets have a weight average size in a range of from 1micron to 7 microns.
 18. The method of claim 1, wherein the dropletshave a weight average size in a range of from 1 micron to 5 microns. 19.The method of claim 1, wherein as generated the aerosol stream comprisesgreater than 1×10⁶ of the droplets per cubic centimeter.
 20. The methodof claim 1, wherein as generated the aerosol stream comprises greaterthan 5×10⁶ of the droplets per cubic centimeter.
 21. The method of claim1, wherein as generated the aerosol stream comprises greater than 1×10⁷of the droplets per cubic centimeter.
 22. The method of claim 1, whereinthe glass particles comprise a dielectric glass composition.
 23. Themethod of claim 1, wherein the glass material is an oxide glass.
 24. Themethod of claim 23, wherein the oxide glass comprises at least: a firstcomponent selected from the group consisting of SiO₂, B₂O₃, P₂O₅ andGeO2; and a second component selected from the group consisting ofAl₂O₃, Bi₂O₃ and PbO.
 25. The method of claim 24, wherein the oxideglass comprises an alkali oxide.
 26. The method of claim 24, wherein theoxide glass comprises at least one alkali oxide selected from the groupconsisting of an oxide of Li, an oxide of Na, an oxide of K, an oxide ofRb and an oxide of Cs.
 27. The method of claim 24, wherein the oxideglass comprises an alkaline earth oxide.
 28. The method of claim 24,wherein the oxide glass comprises at least one alkaline earth oxideselected from the group consisting of an oxide of Mg, an oxide of Ca, anoxide of Sr and an oxide of Ba.
 29. The method of claim 24, wherein theoxide glass comprises an alkali oxide and an alkaline earth oxide. 30.The method of claim 1, wherein the glass material is silicate glass. 31.The method of claim 1, wherein the glass material is borate glass. 32.The method of claim 1, wherein the glass material is phosphate glass.33. The method of claim 1, wherein the glass material is germanateglass.
 34. The method of claim 1, wherein the glass material isaluminosilicate glass.
 35. The method of claim 1,wherein the glassmaterial is borosilicate glass.
 36. The method of claim 1, wherein theglass material is lead borosilicate glass.
 37. The method of claim 1,wherein the glass material is halide glass.
 38. The method of claim 1,wherein the glass material is chalcogenide glass.
 39. The method ofclaim 1, wherein: the glass material is selected from the groupconsisting of silicate glass, borate glass, phosphate glass andgerminate glass; the glass material comprises a component selected fromthe group consisting of Al₂O₃, Bi₂O₃ and PbO; and the glass comprises atleast one component selected from the group consisting of an oxide ofLi, an oxide of Na, an oxide of K, an oxide of Rb, an oxide of Cs, anoxide of Mg, an oxide of Ca, an oxide of Sr and an oxide of Ba.
 40. Themethod of claim 88, wherein the glass comprises multiple componentsselected from the group consisting of an oxide of Li, an oxide of Na, anoxide of K, an oxide of Rb, an oxide of Cs, an oxide of Mg, an oxide ofCa, an oxide of Sr and an oxide of Ba.
 41. The method of claim 1,wherein the weight average particle size is from 0.3 micron to 5microns.
 42. The method of claim 1, wherein the weight average particlesize is not greater than 3 microns.
 43. The method of claim 42, whereinthe weight average particle size is at least 0.05 micron.
 44. The methodof claim 42, wherein the weight average particle size is at least 0.1micron.
 45. The method of claim 1, wherein the glass particles compriseat least 90 weight percent glass.
 46. The method of claim 1, wherein theglass particles comprise at least 95 weight percent glass.
 47. Themethod of claim 1, wherein: the glass particles are substantiallyspherical, have a density of at least 90 percent of the theoreticaldensity, have a weight average particle size of from 0.05 micron to 3microns; and the glass material comprises: (i) at least one componentselected from the group consisting of SiO₂, B₂O₃, P₂O₅ and GeO₂; and(ii) at least one component selected from the group consisting of Al₂O₃,Bi₂O₃ and PbO; (iii) at least on component selected from the groupconsisting of alkali oxides and alkaline earth oxides.
 48. The method ofclaim 47, wherein the glass particles comprise no greater than 0.1atomic percent impurities.
 49. The method of claim 1, wherein thegenerating comprises forming the droplets from a reservoir of the liquidfeed ultrasonically energized by a plurality of ultrasonic transducersunderlying the reservoir.
 50. The method of claim 1, wherein thegenerating comprises forming the droplets from a spray nozzle.
 51. Themethod of claim 1, comprising processing the glass particles, whereinthe processing comprises forming a coating on the particles, the coatingcomprises a coating material that is different than the glass material.52. The method of claim 51, wherein the coating has a thickness of lessthan 100 nanometers.
 53. The method of claim 51, wherein the coating hasa thickness of less than 50 nanometers.
 54. The method of claim 51,wherein the coating has a thickness of between 5 nanometers and 50nanometers.
 55. The method of claim 51, wherein the coating material isan inorganic material.
 56. The method of claim 51, wherein the coatingmaterial is a metal phase.
 57. The method of claim 51, wherein the metalphase is an elemental metal.
 58. The method of claim 51, wherein thecoating material is an organic material.
 59. The method of claim 51,wherein the coating material is a surfactant.
 60. The method of claim51, wherein the coating material is polymethylmethacrylate.
 61. Themethod of claim 51, wherein the coating material is polystyrene.
 62. Themethod of claim 51, wherein the coating material is an inorganicdielectric coating.
 63. The method of claim 51, wherein the coatingmaterial is an organic dielectric coating.
 64. The method of claim 51,wherein the coating material is hydrophobic.
 65. The method of claim 51,wherein the coating material hydrophilic.
 66. The method of claim 51,wherein the coating is a monolayer coating.
 67. The method of claim 51,wherein the forming comprises reacting an organic or inorganic moleculewith the surface the glass particles to form the coating.
 68. The methodof claim 51, wherein the forming comprises reacting the surface of theglass particles with a functionalized organo silane compound.
 69. Themethod of claim 68, wherein the functionalized organo silane compound isa halo-silane.
 70. The method of claim 68, wherein the functionalizedorgano silane compound is an amino-silane.
 71. The method of claim 68,wherein the functionalized organo silane compound ishexamethyidisilazane.
 72. The method of claim 68, wherein thefunctionalized organo silane compound is trimethylsilylchloride.
 73. Themethod of claim 51, wherein the forming comprises contacting the glassparticles with a reactive gas composition.
 74. The method of claim 51,wherein the forming comprises chemical vapor deposition.
 75. The methodof claim 51, wherein the forming comprises physical vapor deposition.76. The method of claim 51, wherein the forming comprisesgas-to-particle conversion.
 77. A method for making a glass layer on asubstrate, the method comprising: making the glass particles accordingto the method of claim 1; depositing the glass particles on a substrate;and sintering the glass particles.
 78. The method of claim 77, whereinthe making comprises heating the particles in a thermal reactor.
 79. Themethod of claim 78, wherein the thermal reactor is a furnace reactor.80. The method of claim 78, wherein the thermal reactor is a flamereactor.
 81. The method of claim 78, wherein the thermal reactor is aplasma reactor.
 82. The method of claim 77, wherein the particles have aweight average particle size of from 0.3 micron to 5 microns.
 83. Themethod of claim 77, wherein the particles have a weight average particlesize not greater than 3 microns.
 84. The method of claim 77, wherein theparticles have a weight average particle size of at least 0.05 micron.85. The method of claim 77, wherein at least about 80 weight percent ofthe particles are not larger than twice the weight average particlesize.
 86. The method of claim 77, wherein at least about 90 weightpercent of the particles are not larger than twice the weight averageparticle size.
 87. The method of claim 77, wherein the glass particlescomprise a complex glass.
 88. The method of claim 87, wherein thecomplex glass is an oxide glass.
 89. The method of claim 88, wherein theoxide glass comprises at least: a first component selected from thegroup consisting of SiO₂, B₂O₃, P₂O₅ and GeO₂; and a second componentselected from the group consisting Al₂O₃, Bi₂O₃, PbO.
 90. Thecomposition of claim 89, wherein the oxide glass comprises an alkalioxide.
 91. The composition of claim 89, wherein the oxide glasscomprises at least one alkali oxide selected from the group consistingof an oxide of Li, an oxide of Na, an oxide of K, an oxide of Rb and anoxide of Cs.
 92. The composition of claim 89, wherein the oxide glasscomprises an alkaline earth oxide.
 93. The composition of claim 89,wherein the oxide glass comprises at least one alkaline earth oxideselected from the group consisting of an oxide of Mg, an oxide of Ca, anoxide of Sr and an oxide of Ba.
 94. The composition of claim 89, whereinthe oxide glass comprises an alkali oxide and an alkaline earth oxide.95. The composition of claim 87, wherein the complex glass is silicateglass.
 96. The composition of claim 87, wherein the complex glass isborate glass.
 97. The composition of claim 87, wherein the complex glassis phosphate glass.
 98. The composition of claim 87, wherein the complexglass is germanate glass.
 99. The composition of claim 87, wherein thecomplex glass is aluminosilicate glass.
 100. The composition of claim87,wherein the complex glass is borosilicate glass.
 101. The compositionof claim 87, wherein the complex glass is lead borosilicate glass. 102.The composition of claim 87, wherein the complex glass is halide glass.103. The composition of claim 87, wherein the complex glass ischalcogenide glass.
 104. The composition of claim 87, wherein: thecomplex glass is selected from the group consisting of silicate glass,borate glass, phosphate glass and germinate glass; the complex glasscomprises a component selected from the group consisting of Al₂O₃, Bi₂O₃and PbO; and the glass comprises at least one component selected fromthe group consisting of an oxide of Li, an oxide of Na, an oxide of K,an oxide of Rb, an oxide of Cs, an oxide of Mg, an oxide of Ca, an oxideof Sr and an oxide of Ba.
 105. The composition of claim 104, wherein theglass comprises multiple components selected from the group consistingof an oxide of Li, an oxide of Na, an oxide of K, an oxide of Rb, anoxide of Cs, an oxide of Mg, an oxide of Ca, an oxide of Sr and an oxideof Ba.
 106. The method of claim 77, wherein the glass particles compriseat least 90 weight percent glass.
 107. The method of claim 77, whereinthe glass particles comprise at least 95 weight percent glass.
 108. Themethod of claim 77, wherein the glass particles comprise no greater thanabout 0.1 atomic percent impurities.
 109. The method of claim 77,wherein the depositing comprises ink-jet printing.
 110. The method ofclaim 77, wherein the depositing comprises delivery of the glassparticles from a syringe.
 111. The method of claim 77, wherein thedepositing comprises delivery of the glass particles from a micropen.112. The method of claim 77, wherein depositing comprises slurrydeposition.
 113. The method of claim 77, wherein the depositingcomprises electrophoresis.
 114. The method of claim 77, wherein thedepositing comprises electrostatic deposition.
 115. The method of claim77, wherein the sintering comprises heating the glass particles. 116.The method of claim 115, wherein the heating the glass particlescomprises heating with a localized energy source.
 117. The method ofclaim 116, wherein the localized energy source is a laser.
 118. Themethod of claim 115, wherein the heating the glass particles comprisesheating with a furnace.
 119. The method of claim 77, wherein thesintering comprises densifying the layer.
 120. The method of claim 77,wherein the substrate is a glass substrate.
 121. The method of claim 77,wherein the substrate is a polymeric substrate.